1. Field of the Invention
The present invention relates to an information recording medium, for example, such as an optical disc, a photo-electro-magnetic disc or the like, and also to an information recording medium reproducing apparatus for reproducing information from the information recording medium.
2. Discussion of the Related Art
In a conventional optical disc used in a CAV (constant angular velocity) mode, a servo byte interval is periodically provided at a predetermined position of each track so that clock pits for generation of reference clocks and wobbled pits for tracking are formed in the servo byte interval. Reference clocks (channel clocks) are generated in correspondence with the clock pits, and information is digitally recorded by pits having a length which is integral number times as long as a period of the reference clock.
On the other hand, for example, in a system used in a CLV (constant linear velocity), for example, such as a CD (compact disc), there is no clock pit, but, a length of recorded pits and an interval of the pits are selected (the so-called self-clock system) so as to have lengths (in the CD, nine kinds of lengths ranging from approximately 0.9 .mu.m to 3.3 .mu.m) which are integral number times as long as a period (0.3 .mu.m) of the reference clock (channel clock). The clock components contained in a reproduced RF signal are extracted therefrom, and recorded information is cut into bit units.
By the way, in the video disc which constitutes the same optical disc, a video signal is modulated by a difference in the length of the pit which is finer by far than that of the CD, recorded and reproduced. Now, this will be described with reference to an example of a signal recorded at a portion of a radius 55 mm in the CAV mode. In the video disc, the brightest portion in the video signal is recorded as a signal of 9.3 MHz whereas the darkest portion therein is recorded as a signal of 7.6 MHz. On a disc having a radius of 55 mm, the former corresponds to 1.075 .mu.m whereas the latter corresponds to 1.316 .mu.m. There is well known such a fact that a fine image can be reproduced when the disc on which the signals have been thus recorded is reproduced.
If a change in brightness of 128 gradations can be expressed, it is meant that the period of pits is changed finely by 128 steps or more on the disc, recorded and reproduced. That is, a change in the fine pit length and the pit distance represented by the following expression is reflected on the video signal. EQU (1.316 .mu.m-1.075 .mu.m).div.128=0.002 .nu.m
In spite of the fact that such a fine change in length of the pit can be recorded, the reason why the minimum unit of the change in the length of the pit must be enlarged to 0.3 .mu.m is mainly because the recording/reproducing method is not optimum.
Applicant(s) has previously proposed in Japanese Patent application No. Hei 3-167585 that the position of the front edge or the rear edge of an information pit is shifted from a predetermined reference position in correspondence with recording information step-by-step to record digital information. According to this recording/reproducing method, as the change in length of the pits and position of a pit edge can be detected with very high accuracy, it is possible to record digital information with a minute change which has been heretofore considered impossible. As a result, it can be realized to make the density higher than till now.
FIGS. 23(a) to 23(c) show a principle of recording information by shifting the position of the edge step-by-step as previously proposed by Applicant(s). As shown in the figures, a recording signal (FIG. 23(B)) which has been subjected to PWM modulation is generated in correspondence with recording data. Then, a pit (FIG. 23(A) corresponding to the length of the recording signal at the time of zero-cross is formed. In this manner, the position of the pit edge is changed step-by-step from a position represented by a reference clock (FIG. 23(C)). In response to the amount of this change, data of 8 gradations (3 bits) ranging from 0 to 7 can be recorded.
FIGS. 24(A) to 24(D) show a principle of reproducing the signal thus recorded. An RF signal (FIG. 24(A)) reproduced by the information recording medium is largely amplified to obtain a binary coded RF signal (FIG. 24(B)). Because the clock pits are in the disc on which information has been recorded, reference clock signals (FIG. 24(C)) are produced on the basis of the clock pits, and saw-tooth wave signals (FIG. 24(D)) are further produced in synchronism with the reference clock signals. Then, the position of the edge of the information pit is detected by detecting a timing at which the saw-tooth wave signals and the binary coded RF signals cross.
Thus, in the case where the position of the pit edge is changed step-by-step in correspondence with the recording data, the clock signal with high accuracy is required as a base of reproduction.
For example, as described above, because of using the self-clock system, all edges of the pits come to have the clock information. As a result, the clock information is recorded on the disc at short intervals, and therefore the clock signal can be readily produced from a clock component contained in the reproduced signal by a PLL circuit. Consequently, in this case, a loop band can be broadened, and even in the case where the rotational speed of the disc is shifted by a relatively large amount from a regular value, the PLL can be locked. For that reason, for example, a spindle motor is rotated by applying a predetermined voltage or the like, and subsequently a PLL circuit is operated. Thereafter, a signal produced by the PLL circuit is used for adjusting the rotational speed of the spindle motor with accuracy.
However, as in the case of the fore-mentioned proposal, in the system where the position of the pit edge is shifted step-by-step in correspondence with the recording data, because the edge has no clock information, clock data is necessary to be recorded separately in a predetermined area on the track.
If a recording interval of the clock data is shortened, as mentioned above, the PLL band for generating the clock signal can be further broadened, and this is advantageous in view of clock generation. However, if the recording interval of the clock data is made narrow, a data region where essential data is recorded and/or reproduced is reduced as much, thereby lowering a recording capacity of the disc. That is, even though the interval at which the clock data is recorded is made narrow, there is a limit thereof in view of the practical recording capacity of the disc.
As a result, the sampling frequency (inverse number of a period) of the clock data in the system in which the position of the pit edge is shifted step-by-step in correspondence with the recording data comes to be lowered by the degree of one digit in comparison with that of a CD. For that reason, it is necessary to make the frequency band of the PLL circuit narrow, with the result that a capture range of the PLL circuit (drawable frequency range) is made narrow. Consequently, to generate the clock signal by using the PLL circuit, it is necessary to accurately adjust the rotational speed of the disc to a predetermined rotational speed which has been previously set.